Comparative measurement of an outer diameter of a workpiece with a certain design dimension initially requires the zero point adjustment of a detector using a master of the dimension. The zero point adjustment must be made each time the dimensions of the measured workpiece are changed, requiring a large amount of time and effort.
A measuring head disclosed in Japanese Patent Examined Application Publication No. 6-48161, as shown in FIG. 8, is mainly composed of a base lever 2 including a detector 1 at a base end thereof, a measurement lever 3 including a contact 3a at a tip end thereof, a rotatable fulcrum shaft 4 pivotally supporting a tip end of the base lever 2 as well as a base end of the measurement lever 3, a clamp mechanism 5 that connects/releases the base lever 2 and measurement lever 3 to/from the fulcrum shaft 4, a setting arm 6 for swinging the base lever 2 relatively to the fulcrum shaft 4, and a manipulating mechanism 7 for actuating the clamp mechanism 5 and setting arm 6. The zero point adjustment for the head is accomplished as described below.
Rotating a handle 8 of the manipulating mechanism 7 causes a manipulating shaft 8a to rotate, and an eccentric cam 9b fixed to the manipulating shaft 8a acts on the clamp mechanism 5 and then the fulcrum shaft 4 is unclamped. The base lever 2 and measurement lever 3 are then rotatably supported relatively to the fulcrum shaft 4. The rotation of the manipulating shaft 8a also causes a regulator plate 6a to move forward with the action of the eccentric cam 9a fixed to the manipulating shaft 8a, and to push the setting arm 6, moving the base lever 2 to the zero point position of the detector 1.
At this point, a master is placed between the contacts 3a and caught by the contacts 3a. Rotating a handle 8 again causes the clamp mechanism 5 to actuate with the action of the eccentric cam 9b, and the base lever 2 and measurement lever 3 are fastened to the fulcrum shaft 4. At the same time, the action of the eccentric cam 9a causes the regulator plate 6a to move backward, releasing the setting arm 8. This makes the head ready for measurement and completes the zero point adjustment.
Although the measuring head with the above arrangement can accomplish the zero point adjustment, it suffers from the disadvantage that it has a complex mechanism and a large number of parts, leading to cumbersome assembly and a large size of the overall head.
A measuring head 100 disclosed in Japanese Patent Application Laid-Open No. 2002-181502, as shown in FIG. 9, is rotatably provided with a base arm 114 in a head body 112 by means of a rotating support shaft 116 as the fulcrum. A swinging support shaft 136 is provided at the tip end of the base arm 114. A measurement arm 138 is swingably supported by the swinging support shaft 136, and the measurement arm 138 can be secured in a position at any angle to the swinging support shaft 136 by means of a clamp mechanism 140. A setting arm 122 is provided on the base arm 114, and pushing the setting arm 122 with a movable plate 124 causes the base arm 114 to move to the zero point position of a differential transformer 118. The zero point adjustment is accomplished by causing the base arm 114 to move to the zero point position of a differential transformer 118, and swingably supporting the measurement arm 138 relatively to the swinging support shaft 136. In this state, a master W is caught between contacts 142, and then the measurement arm 138 is secured by the clamp mechanism 140.
The measuring head with the above arrangement, however, also suffers from the disadvantage that it has a complex mechanism for the zero point adjustment, leading to cumbersome assembly and zero point adjustment.
In view of the circumstances noted above, it is an object of the present invention to provide a measuring head that facilitates the zero point adjustment and is small and simple in its construction.